a) Field of the Invention
The present invention relates to a process for manufacturing pellets of chrysotile asbestos that are particularly useful for the transportation in bulk of this mineral. More particularly, the present invention relates to a process for the manufacturing pellets of chrysotile asbestos, which comprises two basic steps, namely the pelletizing of asbestos fibers followed by the spraying onto the pellets, of a coating that is compatible with the subsequent use of the pellets and reduces to almost zero the release of dust and air-borne fibers during transportation and use of the pellets.
The invention also relates to the coated pellets of chrysotile asbestos fibers obtained by this process.
b) Brief Description of the Prior Art
It is of common practice throughout the world, to transport industrial minerals in bulk. For some applications, it is also of common practice to condition the minerals in order to facilitate their handling. As an example of such a conditioning, reference can be made to the pelletizing of iron ore. As a matter of fact, pelletizing which is also called "agglomeration", is a widely used technique for the treatment of fine iron ore concentrates, which otherwise could not be easily handled, shipped or charged to a blast furnace. This technique is also used for the production of direct reduced iron.
In this connection, reference can be made to U.S. Pat. application Ser. No. 07/844,281 filed on Mar. 2, 1992, in the name of the Applicant, which discloses a self-fluxing binder composition for use in the pelletizing of ore concentrates, comprising a mixture of a fluxing material selected from the group consisting of natural and synthetic magnesium or calcium bearing minerals, and a water soluble organic substance selected amongst natural polysaccharide of high viscosity.
As aforesaid, pelletizing is a technique that has been successfully applied to a wide variety of materials, such as fertilizer, coal chemicals, nonmetallic minerals, industrial wastes and the like to make handling, shipping or use of these materials much easier to carry out.
Pelletizing has also been used on chrysotile asbestos, to make this mineral easier to handle.
Thus, for example, U.S. Pat. No. 3,623,858 granted on Nov. 30, 1971 to JOHNS-MANVILLE CORPORATION discloses a method for pelletizing serpentine chrysotile fines by addition thereto of an aqueous solution containing 5 to 10% by weight of a mineral acid which is preferably sulphuric acid, and subsequent accretion of this mixture into pellets.
Canadian patent No. 1,244,817 granted on Nov. 15, 1988 to INSTITUT DE L'AMIANTE discloses, in its example 1, the formation of chrysotile asbestos pellets for use as a filter to remove cancer-causing compounds from exhaust fumes. The pellets are produced by addition of bentonite and sufficient water to wet the dry fibers, and subsequent pelletizing of the resulting mixture in a rotating disk pelletizer. After they are formed, the pellets are dried to harden them and make them dust free.
U.S. Pat. No. 5,028,266 granted on Jul. 2nd, 1991 to Mr. Stephan RETTENMAIER discloses the formation of granulates of chrysotile asbestos for use as an additive in a liquid bitumen mass. In a first step, the fibers are coated with a binding agent which is preferably bitumen. Then, the coated fibers are agglomerated. The resulting granules are intended to be admixed with liquid bitumen until they are dissolved in it.
The above-mentioned prior art clearly shows that pelletizing of chrysotile asbestos fibers with water and a binder is a known technique. However, to the Applicant's knowledge, the prior art in this field has never addressed what is now the major issue in the transportation, handling and use of chrysotile asbestos, namely reduction and, ideally, elimination of the release of asbestos dust or air-borne asbestos that can be a health hazard for those in contact with this mineral, even though there exists a substantial number of publications dealing with this problem and suggesting different ways of eliminating or substantially reducing the release of asbestos dust and particles in the ambient air during handling this mineral. On this matter, reference can be made, for example, to the "Guidelines for the use of encapsulants of asbestos-containing materials" published on June 1981 by the Chemical Control Division of the EPA's Office of Toxic Substances, with the help of Mr. William MINICK of the Batelle Columbus Laboratories.